Datasheet

LTC3766
18
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
Secondary-Side Bias and Start-Up
In most applications, the LTC3766 will receive its bias
voltage from a supply that is generated on the secondary
side. The manner in which the secondary bias is generated
depends upon the output voltage as well as the variation in
the input voltage of the DC/DC converter. In all applications,
however, the secondary bias must always come up before
the output reaches the regulation level. This is essential
to avoid an overvoltage condition on the output, since the
initial start-up is performed from the primary side in an
open-loop fashion. See Generating the Secondary-Side
Bias for more information.
Note that the LTC3766 will not begin a soft-start sequence
and initiate switching until the RUN pin is high, adequate
voltage is present on both the V
IN
and V
CC
pins, and
switching is detected on the SW pin. The LTC3766 looks
for switching on the SW pin to ensure that the LTC3765 is
active and ready for control hand-off. For switching to be
detected, the SW node waveform must have at least eight
consecutive pulses in the range of 50kHz to 700kHz. The
SW node waveform must also have a peak that is greater
than 1.4V and a valley that is less than 0.5V. In standalone
mode, the LTC3766 begins the soft-start sequence without
waiting for a switching waveform to be detected on the
SW pin.
Linear Regulator Operation
The LTC3766 contains two linear regulators that are used to
regulate the available bias voltage down to a level suitable
for driving MOSFETs. If the bias supply voltage is greater
than 15V, then the high voltage linear regulator controller
may be used. This makes use of an external N-type pass
device. Place a capacitor of 0.22μF or greater on V
IN
and
1μF or greater on V
CC
. If the bias supply connected to the
V
IN
pin has a relatively high output impedance, it may be
necessary to use a larger capacitor on V
IN
to prevent the
V
IN
pin voltage from dropping when the V
CC
capacitor is
being charged. The V
CC
charge rate during linear regulator
start-up is set by the LTC3766 to approximately 0.5V/μs,
which will create at charging current of (0.5 • 10
6
) C
VCC
.
Care should be taken to ensure that this charging current
does not exceed the SOA of the N-type pass device, par
-
ticularly when operating at higher V
IN
voltages. The V
CC
regulation level can be set to either 7V or 8.5V as desired
using the MODE pin. See the section on V
CC
and Drive
Mode Selection for details.
The LTC3766 also contains a 15V internal bypass LDO. If
the voltage on the V
AUX
pin exceeds the V
AUX
switchover
threshold, then the high voltage linear regulator is disabled,
and an internal PMOS-pass LDO uses the V
AUX
voltage to
supply power to V
CC
. This allows the high voltage linear
regulator to be used for initial start-up and the higher ef-
ficiency bypass LDO to be used during normal operation.
Figure 3 illustrates such a configuration that
uses both
linear regulators.
If the voltage on the V
AUX
pin is below the switchover
threshold, then the V
AUX
pin is internally loaded with a
resistance of approximately 920Ω. This internal load is
removed after the V
AUX
regulator is enabled, and is used
to ensure that the V
AUX
supply is reasonably stiff before
the bypass regulator is activated.
In some cases, it is desirable to use the high voltage linear
regulator only briefly during start-up, so as to limit the
temperature rise in the external pass device. To accomplish
this, place a capacitor on the REGSD pin to ground (see
Figure 3) such that:
C
RSD
=
t
HVREG
13µA
( )
1.21V
where t
HVREG
is the time that the high voltage regulator
will operate. When the high voltage regulator is operat-
ing, a 13μA current is sourced from the REGSD pin, and
when it is shut down (e.g., the bypass regulator is ac-
tive), a 3μA current is sinked into the REGSD pin. If the
REGSD voltage exceeds 1.21V
, the high voltage regulator
is disabled. Choose a time t
HVREG
that is greater than the
normal start-up time. After start-up, if the voltage on the
V
AUX
pin drops, the high-voltage linear regulator will be
re-energized, but only for a limited time.
V
IN
NDRV
LTC3766
OPTIONAL
V
CC
C
VAUX
C
VCC
3766 F03
C
VIN
C
RSD
V
AUX
REGSD LV BIAS
SUPPLY
5V TO 15V
HV BIAS
SUPPLY
6V TO 32V
Figure 3. Typical Linear Regulator Connections